Her biography remains obscure. Her name according to some sources is Émilie d'Eymard: she signed her first letters as Émilie d'Eymard but her first publications as Claire Démar. Her birth date of 1799 is also uncertain, She died in 1833. Her father may have been the pianist and composer of German origins Sebastian Demar, with her mother being Elisabeth Riesam, also of German origin . They had settled in Orléans in 1791, but apparently no birth was recorded in that city's vital statistics for the year 1799 (Year VII-Year VIII, nor in the corresponding decennial table. The couple's daughter named Demar Theresia (Thérèse), harpist and composer (who was her elder sister) was born in Gernsbach, (Germany) in 1786.
Démar was one of the most combative women of Saint-Simonian movement. She used the Saint-Simonian movement to go further and express findings and claims that were rejected by a majority of her contemporaries, but became accepted by feminists during following years. Shortly before her death, she published a Appel d'une femme au peuple sur l'affranchissement de la femme ("Appeal of a woman to the people on the enfranchisement of women") which calls for the application to women of the Declaration of the Rights of Man and Citizen. She also describes marriage as legalized prostitution.
During the last years of her short life, Claire Demar participated in feminist journals created during the opportunity offered by the revolution of 1830, She became associated with the feminist journalist Suzanne Voilquin in her publications La femme nouvelle, L'Apostolat des femmes, et La Tribune des femmes.
Démar was preparing to publish a second book, when, abandoned by all, reduced to the greatest misery, and despairing to see the emancipation of women, she chose suicide together with her lover Perret Desessarts. They were found on the same bed with two letters and a roll of paper, which she had asked to be read in the Saint-Simonian society of Paris and then given to Barthélemy Prosper Enfantin, who sent the papers to Suzanne Voilquin who published them in La Tribune des femmes.